In order to substantially increase the track density in magnetic recording disk files (or "drives"), it is necessary to incorporate a servo control system to center and maintain the head over the tracks during read or write operations. The movement of the head to a desired track is referred to as track accessing or "seeking", while the maintaining of the head over the centerline of the desired track during read or write operations is referred to as track "following". While the need for a servo control system for track following exists in any disk file with a relatively high track density, it is especially critical in the case of flexible disks because such disks are subject to nonuniform distortion due to temperature and humidity changes, spindle motor runout and other effects.
There are several known servo control techniques which utilize the "time of flight" of various servo patterns past the read/write head to indicate the position of the head relative to the centerline of the desired track. In one such technique, as disclosed in U.S. Pat. No. 3,812,522 to Kimura, et al., the servo signals for a group of data tracks are angularly staggered so that the time between a reference pulse and a servo signal in the desired track identifies that track within the group of tracks. This technique is used on disk files using a rigid disk which incorporates prerecorded servo signals on equally angularly spaced sectors which extend out radially from the disk center. As the disk rotates, the head receives sampled track position signals as the servo sectors pass beneath the head. While the use of prerecorded sector servo signals and the technique taught by Kimura to utilize those sector servo signals are applicable to rigid disk files, they are not generally suitable for use with flexible disk drives because flexible disks are generally available only in unrecorded form. Any recording of servo information in angularly spaced sectors would significantly increase the cost of the flexible disks. Furthermore, the time required to record servo information in multiple sectors on each data track would be prohibitive if recorded by the user's disk drive.
U.S. Pat. No. 4,149,198 to Behr, et al., discloses a disk file head positioning system which utilizes two dedicated servo tracks and associated dedicated positioning servo heads radially spaced on opposite sides of the read/write head. The servo heads and the information recorded in the servo tracks are both slanted relative to the tracks. The time of arrival of pulse trains caused by the two servo heads reading servo information on their respective servo tracks are combined to generate a signal indicative of the position of the read/write head relative to the data tracks.
An article entitled "Servo System for Magnetic Recording Based On Time Comparison" by E. G. Gruss, et al., IBM Technical Disclosure Bulletin, Vol. 23, No. 2 (July 1980), pp. 787-789, discloses the use of intersecting servo markings slanted relative to the data tracks. A comparison of the times of intersection of the head with the servo markings indicates the position of the head relative to the track centerlines.
U.S. Pat. No. 4,346,413 to Hack, U.S. Pat. No. 4,454,549 to Pennington and U.S. Pat. No. 4,488,187 to Alaimo all disclose servo patterns slanted relative to the tracks and various means, other than time of flight measurement, to determine the position of the head relative to the track centerlines.
Co-pending application Ser. No. 713,139, assigned to the same assignee as this application, discloses the use of a servo pattern comprising a group of servo segments slanted relative to the data tracks in which each segment consists of magnetic transitions which increase in frequency in the radial direction. The position of the head relative to the centerline of the desired data track is determined not by the time of flight of the servo segment, but by the number of magnetic transitions actually read by the head.